The invention relates to a process for preparing alcohols by hydrogenation of aldehydes, in which a feed mixture comprising at least one aldehyde and at least one accompanying component is contacted with a heterogeneous catalyst in the presence of hydrogen, giving a product mixture comprising at least the alcohol corresponding to the hydrogenated aldehyde and at least one by-product, wherein the catalyst comprises a support material and copper applied thereto.
The invention further relates to the preparation of the corresponding catalyst and the corresponding precursor, optional activation of the precursor and use of the active catalyst in the process.
Elimination of hydrogen (dehydrogenation) from an alcohol gives rise to an aldehyde. Conversely, alcohols can be prepared from aldehydes by hydrogenation (addition of hydrogen).
Hydrogenation in general is a reaction conducted very frequently in industry. Another specific reaction practised on the industrial scale is the hydrogenation of aldehydes, namely in the preparation of what are called oxo process alcohols.
Oxo process alcohols are alcohols which are prepared by way of hydroformylation (oxo reaction). In hydroformylation, an olefin (alkene) is reacted with a synthesis gas (a mixture of carbon monoxide and hydrogen) to give an aldehyde. Subsequent hydrogenation gives the actual oxo process alcohol. Oxo process alcohols serve as intermediates for the production of surfactants and/or plasticizers for plastic. Several million metric tons of oxo process alcohols are produced globally per year.
Since the hydrogenation of the aldehydes obtained by the hydroformylation is a necessary step in the preparation of oxo process alcohols, the present invention is concerned with a process of relevance on an industrial scale.
In industrial practice, oxo process aldehydes are generally hydrogenated in the liquid phase over heterogeneous fixed bed catalysts. On account of the large throughput volumes, the catalyst is of crucial importance for the process, since it determines the reaction rate and also the selectivity of the hydrogenation. The selection of a suitable catalyst is not trivial since the aldehydes to be hydrogenated never occur in pure form, but as a mixture of structurally isomeric aldehydes which is always accompanied by a large number of troublesome accompanying components which firstly bring about secondary reactions undesired in the hydrogenation and secondly damage the hydrogenation catalyst. Since the composition of the feed mixture comprising the aldehydes to be hydrogenated is determined by the upstream hydroformylation, the hydrogenation catalyst has to be exactly adjusted with respect to the particular hydroformylation.
For the hydrogenation of oxo process aldehydes, useful catalysts have been found to be those comprising a support material to which copper, chromium and nickel have been applied as active components.
A corresponding catalyst is disclosed in DE19842370A1. It comprises copper and nickel, each in a concentration range from 0.3% to 15% by weight and chromium in a proportion by weight of 0.05% by weight to 3.5% by weight. The support material used is porous silicon dioxide or aluminium oxide.
U.S. Pat. No. 4,677,234 describes a process for preparing ethylene glycol in the presence of a supported copper catalyst.
Although these catalysts have proven useful in the industrially practised hydrogenation of oxo process aldehydes, there is still a need for an alternative. The reason for this is the chromium content of these catalysts.
According to Annex XIV of the REACH directive, chromium-containing substances such as the catalysts described above must only be used in the European Union after authorization by the Commission. The granting of authorization is associated with great complexity and high costs; moreover, granting of authorization cannot be expected a priori. Moreover, the application procedure has to be repeated every five years.
The reason for these strict conditions is the undisputed carcinogenicity of the chromium used. This is of relevance firstly when the hydrogenation catalyst has to be disposed of following deactivation, and secondly when it is newly produced by impregnation with alkali metal chromates or alkali metal dichromates.
The chromium problem has been solved with the catalyst disclosed in EP3037400A1, which is virtually chromium-free. However, there is further need for improvement in this system, since nickel and the nickel compounds used in the production of the chromium-free catalyst are likewise carcinogenic.
In this respect, the problem addressed is that of specifying a catalyst system suitable for industrial hydrogenation of aldehydes, which is both free of chromium and free of nickel.
EP2488478B1 describes a two-stage hydrogenation of C10 aldehydes, wherein a catalyst which is free of copper, chromium and nickel but does contain ruthenium is used in the second hydrogenation stage. Ru is comparatively costly, and for that reason this process is not always economically viable on the industrial scale. Furthermore, the process is not nickel-free either, since a nickel-containing catalyst has to be used in the first stage in order to achieve acceptable hydrogenation results.
WO95/32171A1 describes various hydrogenation catalysts comprising copper and silicon dioxide, either in the presence or absence of further elements including chromium. The specific chromium-free variants are notable for very high CuO contents (well above 20% by weight). The raw material costs for such copper-rich catalysts are quite high.
U.S. Pat. No. 3,677,969 describes an organometallic hydrogenation catalyst. A disadvantage of this system is that the production thereof is comparatively costly since it entails an additional sulphidation and it has to be heat-treated at very high temperatures (400° F. to 1000° F.). Moreover, an optional content of chromium and nickel is recommended.
In view of all the above, it has not been possible to date to find a chromium- and nickel-free catalyst suitable for the hydrogenation of hydroformylation mixtures on the industrial scale.